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Compare the measured electricity or electrical parameters with electrical standards or provide accurate proportional measuring instruments. A large number of measuring instruments (abbreviated as comparative measuring instruments) used in scientific research, quantity transfer, and industrial testing are all electrical measuring instruments. It has a wide range of uses and occupies an important position in the field of metrology and testing.

Electrical measuring instruments mainly use proportional technology to achieve measurement. For direct current, it is the voltage ratio formed by the voltage generated by the same current on two resistors, or the current ratio of two resistors at the same voltage, and then combined with a standard to measure unknown quantities. A device that provides proportions is like a balance, while a standard device is like a weight. The measuring instruments made based on this analogy include DC bridges, DC potentiometers, etc. For alternating current, the measurement principle is basically the same as for direct current, except that the resistance is replaced by impedance. Therefore, in general, the proportion is plural; The real or imaginary ratio is only a special case. In addition, two coils with magnetic coupling can also be used to obtain a voltage real ratio proportional to the number of turns, or a current real ratio inversely proportional to the number of turns. The measuring instruments made based on these principles include classic AC bridges, inductively coupled proportional arm bridges, AC potential difference meters, induction voltage dividers, current comparators, transformers, etc. In addition to the aforementioned electrical measurement instruments, there are also equivalent circuit components composed of electronic circuits, as well as active and digital bridges made using digital technology. Since the 1970s, the combination of measuring instruments and microcomputers has expanded its functions and developed towards intelligence.